1. Field of the Invention
The field of the invention relates generally to growing single crystal semiconductor material by the Czochralski process. In particular, the field of the invention relates to a continuous Czochralski process employing a tall weir comprising optionally a second weir providing a double weir. The tall weir extends vertically and is configured for providing an optimal flow and velocity of an inert gas, such as argon, to be directed away from the crystal growth region to the melt region, thereby preventing airborne particles or other contaminants from causing dislocations or related defects in the growing single crystal structure. The double weir's function is to properly melt unmelted particles of silicon by ensuring sufficient transit time and optimal crystal growth conditions.
2. Background of Related Art
In a continuous Czochralski (CZ) crystal growth the melt is supplemented at the same time that the crystal is growing. This is in contrast with batch recharge wherein the melt is recharged after the melt is depleted by a completion of a crystal growing and pulling process. In either case the melt can be supplemented either with solid feedstock or molten feedstock.
In contrast to batch recharge, there are advantages of a continuous Czochralski process for growing single crystal silicon ingots. The melt height remains constant and therefore the growth conditions at the melt/crystal interface can be maintained more uniformly for optimal crystal growth. The cycle time is reduced because the melt conditions are not suddenly changed by the addition of a large quantity of feedstock.
In the case of batch recharge, increased cycle time is particularly significant when feedstock must be added in solid form, and this disadvantageously can add up to several hours of equilibration time to the growth cycle. Addition of liquid feedstock particularly in batch recharge reduces the melt equilibration time, but involves the considerable design complications and increased cost of a pre-melter.
A disadvantage of a Czochralski continuous or batch recharge crystal growth is that silicon dust and un-melted particles of silicon, whether from a pre-melter or resulting from the addition of solid feedstock into the melt, are transferred into the melt and growth chamber and can become attached to the growing ingot causing it to lose its single crystal structure.
Silicon feedstock consists of solid chunks, chips or granules that abrade against each other to form small particles and dust. Particularly when solid feedstock is added to the melt, the small particles and dust can be carried into the growth chamber to become suspended in the atmosphere of low pressure argon. The particles and dust can then fall into the melt and cause dislocations in the growing ingot. Smaller size distributions in the solid feedstock are preferred. Adding smaller size chips of feedstock to the melt results in less melt disturbance and splashing. Smaller size feedstock exhibits more controllable flow characteristics in the solid state. In the solid state, smaller particles exhibit more uniform melting characteristics. However, smaller size chips of feedstock are characterized by greater surface area to volume. Thus, the preferred use of such small feedstock material exacerbates the problems of dust. The preferred size distribution is 1-19 mm or more preferably 1-12 mm for small Siemens chip material or 1-3 mm for granular fluidized bed reactor (FBR) material. Unfortunately, these small size distributions are characterized by relatively high levels of small particles and dust which readily can become airborne.
At silicon process temperatures, silicon oxide formed by dissolution of the quartz crucible evaporates from the melt and condenses on slightly cooler areas of the hot zone to form a powder or dust that may become a serious maintenance problem. When this powder or dust falls back into the silicon melt it may affect the growing single crystal structure, causing dislocation defects. Ingot yield and growth economics suffer severely. Therefore, what is needed is a method and apparatus for substantially eliminating airborne particles that may cause defects in the growing crystal.
In a conventional process for continuous crystal growth a weir is often used. The weir typically consists of a quartz tube section, positioned vertically on the floor of the crucible and extending above the melt surface defining a first inner growth region and a second outer melt supplementing region with one or more subsurface passageways connecting the foregoing first and second melt regions. Such a conventional weir arrangement is shown in FIG. 1.
While this arrangement may be adequate for limiting transmission of un-melted particles of silicon from the melt supplementing region to the crystal growth region, such conventional weir arrangements fail to address the problem of dust particles which can be carried over the weir by aerostatic forces, or otherwise become airborne, and land in the growth region with destructive effects on crystal structure.
The problem of silicon dust adversely impacting the conventional CZ crystal growing process is expected to increase in severity as the solar power industry turns to processing of recycled silicon.
Solar cell production is presently constrained by the high cost and lack of availability of input polysilicon raw material. The single crystal ingot grown in a Czochralski process is necessarily grown with a conical top and conical tail. Pot scrap is necessarily left in the crucible when the ingot is removed. Furthermore, the round cross section of the ingot body must be converted to a square or pseudo-square cross section to provide silicon wafers in the form of tiles for optimal area coverage in a solar module. Thus, ear or slab shaped sections must be removed from the ingot to create the square or pseudo-square cross section. These bi-product materials must be recycled for lowest possible wafer cost, and in so doing they must be broken up and re-sized into small chunks or chips. The breaking up process necessarily creates small particles and dust.
Therefore, what is needed is an improved CZ grower that is compatible with silicon recycling processes and is capable of using the silicon produced from re-sized bi-product materials. Such an improved CZ grower ideally must be capable of economically processing recycled silicon, such as tops, tails, slabs and pot scrap while substantially eliminating the problem of unmelted particles and dust landing in the growth zone to cause dislocation defects in the growing crystal.
Other problems of conventional CZ growers also adversely impact their ability to economically process recycled silicon to produce high purity silicon ingots free of dislocation defects that are demanded by higher efficiency solar cells. For example, conventional weirs, being made from quartz, are subject to the formation of silica particulates that can float away from the surface of the quartz into the growth region and destroy crystal structure. Quartz softens at temperatures commonly encountered in a crucible for holding molten silicon. Conventional quartz weirs deform, thereby leading to operational problems.
A further problem of conventional uncoated weirs is that melt vibrations induced by interaction between the melt and weir walls causes dislocations in the growing ingot.